This invention relates to a casting composition for use in the construction of troughs, runners, ladles and other vessels which are used for containing and processing molten iron and steel. When compared with known casting compositions, the composition of the invention requires much less drying time and is less prone to cracking or explosion during drying.
The composition of the invention can be prepared by mixing 55-90% by weight of a refractory base material, such as calcined clay, mullite, brown fused alumina or tabular alumina, with up to 35% by weight of silicon carbide and 8-14% by weight of a binder material which includes very fine (preferably colloidal) silica particles dispersed in water.
Brown fused alumina is a term of art which refers to a dark brown, glassy looking material including approximately 93-98% by weight alumina, and smaller amounts of titania, silica and iron. Tabular alumina is a term of art which refers to a white, opaque material including approximately 98.5-99.9% by weight alumina and smaller amounts of sodium oxide, silica, magnesium oxide and calcium oxide. Calcined clay is a term of art which refers to a composition having approximately 43-48% by weight alumina, 47-50% by weight silica, and smaller amounts of titania, iron and sodium oxide. Mullite is a term of art which refers to a composition containing approximately 57-73% by weight alumina, 27-40% by weight silica, and smaller amounts of impurities.
Preferably, the composition will also include between 0.02-1% by weight of a setting agent such as magnesium oxide or calcium aluminate cement, 5-20% by weight of calcined alumina, and 1-10% by weight of microsilica. Calcined alumina is a term of art which refers to a composition containing approximately 99.6% by weight of alumina and trace amounts of sodium oxide and silica. The primary difference between calcined alumina and tabular alumina is that calcined alumina has a lower firing temperature (about 2600.degree.-2700.degree. F.) than tabular alumina (about 3400.degree. F.). As a result, calcined alumina is finer and more reactive than tabular alumina.
Between 2-10% by weight of a graphite material may optionally be included as a nonwetting agent and to inhibit the chemical reaction between "slag" (present in molten steel) and the refractory base material.
The use of a colloidal silica binder represents an improvement over known casting compositions which utilize calcium aluminate cement and/or clay. Calcium aluminate cement and clay chemically bind to water. Therefore, troughs and runners produced from compositions which include these binder materials are relatively difficult to dry, requiring between 30-80 hours of drying time at temperatures of up to 1500.degree. F., depending upon the thickness of the structure. Furthermore, the structures may crack or even explode during drying at temperatures above 250.degree. F. Complete removal of residual water is an essential step in the production of troughs and runners, partly to prevent cracking and explosion during use when the structures are exposed to molten iron and steel.
Troughs and runners produced from casting compositions which utilize colloidal silica binders require much less drying time because the silica does not become chemically attached to residual water. Typically, these structures become set after 15 minutes-5 hours at room temperature, followed by 5-30 hours during which time the structure is heated to the desired temperature (i.e., above 250.degree. F.) and completely dried. The amount of setting time required depends on the thickness of the structure. Furthermore, the risks of cracking and/or explosion are substantially eliminated when a colloidal silica binder is used. The colloidal silica binder also imparts several other advantages to the trough or runner including increased strength and improved resistance to oxidation, corrosion and erosion.
An additional advantage of the casting composition of the invention is that it has improved flow characteristics which allow it to be transported to the casting mold using a standard concrete pump. This constitutes a major advantage over casting compositions of the prior art which, due to relatively poor flow characteristics, had to be transported to the mold using a hopper and crane. By using a concrete pump of a type well known for transporting cement, the amount of time required to transport the casting composition and fill the mold in a typical application can be reduced from between 6-8 minutes to about 30 seconds.